HK1227987B - Heat exchanger tube and heating boiler having such a heat exchanger tube - Google Patents

Description

Heat exchange tube and heating boiler having the same

Technical Field

The present invention relates to a heat exchange tube for a heating boiler, in particular a heat exchange tube for a condensing boiler, comprising an outer tube and a profiled insert pushed into the outer tube, wherein exhaust gas from combustion in the boiler can flow through the outer tube and hot water can surround the outer tube on the outside; the profiled insert has ribs extending in its longitudinal direction in order to increase the inner surface of the outer tube and is in heat-conducting contact with the outer tube.

The invention also relates to a heating boiler, in particular a condensing boiler, for heating hot water in a heat circuit, comprising a housing which defines a hot water chamber and has a combustion chamber connected upstream of the hot water chamber.

Background Art

Heating boilers of the type mentioned above, as publicly available for sale by the applicant, can be operated as condensing boilers, either using gas or liquid fuel (heating oil, kerosene, or the like). In these condensing boilers, the gas is also cooled until the exhaust gas moisture condenses, allowing the heat of condensation to be utilized. This presupposes that the heating boiler or condensing boiler is operated with hot water at a temperature that is lower than the dew point of the gas at the end of the gas path through the heating boiler. The goal is to cool the gas from a high inlet temperature to a temperature between the dew point and the lowest hot water temperature present in the heating boiler's hot water circuit, over the shortest possible gas path, using water-cooled heat exchanger tubes in the heating boiler. For this purpose, various heat exchanger tubes are known, with heat exchanger tubes of the type mentioned above being known, for example, from EP 0 752 088 A1.

Summary of the Invention

The object of the present invention is to provide a solution which provides heat exchange tubes and a heating boiler in a structurally simple and inexpensive manner, which enable a greater heat exchange capacity from gas to hot water in the heating boiler.

In a heat exchanger tube of the type mentioned at the outset, this object is achieved according to the invention in that a first longitudinal section of the outer tube is designed in the form of a cylindrical smooth wall, a second longitudinal section of the outer tube has at least one cross-section constriction element which constricts the throughflow cross section, and the profile insert extends only over the first longitudinal section of the outer tube. In other words, the profile insert is arranged only in the first longitudinal section.

In a heating boiler of the aforementioned type, this object is also achieved according to the invention by having at least one heat exchange tube arranged within the housing, which extends from the combustion chamber and through the hot water chamber. While at least one heat exchange tube is conceivable for very low outputs of approximately 10 kW, in many applications, multiple heat exchange tubes are provided. The at least one heat exchange tube can extend, for example, vertically or horizontally through the hot water chamber, with any other angle between 90° (vertical) and 0° (horizontal) also conceivable.

In the following, advantageous and suitable embodiments and developments of the invention are described.

The present invention provides heat exchange tubes and heating boilers having a plurality of such heat exchange tubes, each characterized by a structure that meets functional requirements and has a simple and inexpensive construction. A problem with heat exchange tubes known from the prior art is that hot gas flows through the heat exchange tube from its inlet to its outlet and cools there. The associated significant reduction in gas volume leads to a significant reduction in flow velocity and turbulence up to the heat exchange tube outlet, which negatively impacts heat exchange performance. In contrast, the present invention reduces the pressure loss upstream of the cross-sectional constriction element (i.e., between the combustion chamber and the cross-sectional constriction element) by at least one cross-sectional constriction element that narrows the flow cross-section of the outer tube. As a result, according to the present invention, significantly more energy can be exchanged in the combustion chamber and in the second longitudinal section of the heat exchange tube upstream of the cross-sectional constriction element. The reduced flow cross-section in the longitudinal section upstream of the cross-sectional constriction element significantly increases the exhaust gas flow velocity, thereby increasing not only heat exchange but also the energy efficiency of the exhaust gas. In the longitudinal section downstream of the constriction of the flow cross section (i.e., downstream of the cross-sectional constriction element), the exhaust gas expands again and is guided through the longitudinal section of the outer tube having the profile insert. Due to the very large surface area of the ribs extending in the longitudinal direction of the heat exchanger tube, the exhaust gas is cooled to below the dew point within the first longitudinal section of the outer tube, which benefits condensing boiler technology and, therefore, the performance of the heating boiler. The advantages achieved by the heat exchanger tube and the heating boiler equipped with it according to the present invention can be described as follows: Compared to heat exchanger tubes without a constriction, the increased pressure drop upstream of the constriction leads to better heat exchange within the combustion chamber and at the heat exchanger tube inlet. Furthermore, the increased flow velocity in the region of the constriction, and particularly downstream of the constriction, leads to better heat exchange because the cross-sectional constriction element converts the laminar flow upstream of the constriction into a turbulent flow downstream of the constriction. Finally, the increase in the heat exchange surface achieved by the ribs of the profiled insert leads in the first longitudinal section of the heat exchange tube to low flow velocities downstream of the constriction and to low exhaust gas temperatures, which also contributes to improved heat exchange of the hot water.

In one embodiment of the heat exchanger tube according to the invention, the invention provides that the at least one cross-sectional constriction element is designed as at least one indentation in the wall of the second longitudinal section of the outer tube. This eliminates the need for producing and assembling additional components in order to achieve an advantageous operating mode for the cross-sectional variation.

In one embodiment of the heat exchanger tube according to the invention, it has proven particularly effective that at least one cross-sectional constriction element comprises at least two first indentations, which are formed in the wall of the second longitudinal section of the outer tube, wherein the two first indentations are arranged diametrically opposite each other and are constructed mirror-symmetrically with respect to the first tube plane.

In order to increase the flow velocity downstream of the constrictions, according to another embodiment, at least one first throughflow gap is formed between at least two first constrictions, which gap is between 2% and 3% of the diameter of the outer tube.

In order to further improve the efficiency of the cross-sectional narrowing portion provided for by the present invention, it is provided in one design of the heat exchanger tube that, in addition to the at least two first narrowing portions, the cross-sectional narrowing element also includes at least two second narrowing portions, which are formed by the wall of the second longitudinal section of the outer tube, wherein the two second narrowing portions are arranged diametrically opposite each other and are constructed in a mirror-symmetrical manner with respect to a second tube plane extending perpendicular to the first tube plane.

Furthermore, the invention provides in one embodiment of the second constrictions of the cross-sectional constriction element that at least one second flow gap is formed between at least two second constrictions, which second flow gap is between 18% and 22% of the outer tube diameter.

In order to increase the flow velocity and turbulence downstream of the cross-sectional constriction element, the present invention provides in another embodiment that the first and second bulges formed by the first and second setbacks, which bulge inwardly toward the outer tube, are formed at the same axial position of the second longitudinal section of the outer tube, wherein the flow cross section of the second longitudinal section of the outer tube formed by the first and second bulges has an H-shaped cross section. It is of course also conceivable that the first and second bulges are formed axially offset at different axial positions of the second longitudinal section of the outer tube.

It has proven optimal for the heat exchange tube according to the present invention if, according to one embodiment of the present invention, the axial length of the first longitudinal section corresponds to at least twice the axial length of the second longitudinal section. In an alternative embodiment, the axial length of the second longitudinal section can be greater than the axial length of the first longitudinal section.

The present invention stipulates in an advantageous design that the profiled insert comprises a tube body, which is composed of at least two shell elements, each of which has a fan-shaped cross section. Through this design, the heat exchange tube can be cheaply and therefore can utilize a simple production process to manufacture.

Particularly advantageously, in one embodiment of the heat exchanger tube according to the invention, the tube body comprises two shell elements, which are formed with groove-shaped recesses and rib-shaped projections on their mutually contacting longitudinal edges and thus engage in a sealing manner with one another. The two shell elements are formed on their inner sides with ribs extending into the inner cross section of the tube body and in the longitudinal direction of the outer tube, so that each shell element forms a one-sidedly open shape with its ribs. This embodiment of the shell elements as two half-shells with ribs (as a one-sidedly open shape) can be manufactured simply and inexpensively, for example by extrusion.

In one embodiment of the heat exchanger tube according to the invention, the invention provides that the two shell elements each have a sealing groove on one longitudinal edge and a sealing rib adapted to the shape of the sealing groove on the other longitudinal edge. This labyrinth seal-like design prevents the formation of gaps in the first longitudinal section of the outer tube, through which exhaust gas or condensate could penetrate between the profile insert and the outer tube and potentially cause corrosion.

The heat exchanger tube according to the present invention can be manufactured simply and cost-effectively due to the following design: at least one cross-sectional constriction element is formed as a nozzle-like tube insert that is inserted into the outer tube into its second longitudinal section. This eliminates the need for further machining of the outer tube to account for the indentation or indentation of the cross-sectional constriction element. Instead, it is sufficient to manufacture a separate cross-sectional constriction element with a diameter that matches the inner diameter of the outer tube. This separate cross-sectional constriction element can then be inserted into the outer tube together with the profile insert during assembly or transport of the heat exchanger tube.

In another embodiment of the present invention, the outer tube is formed from a metal alloy, preferably steel, and the profiled insert is formed from aluminum. Due to this material selection, the outer tube is protected from acid and alkali corrosion by exhaust gas condensate. Furthermore, the outer tube can be welded at its ends into the tube base or tube sheet. This separates the hot water chamber surrounding the heat exchange tubes from the combustion chamber, on the one hand, and from the exhaust gas plenum of the heating boiler arranged below the hot water chamber, on the other hand.

Finally, to increase the heat exchange performance, the present invention provides that a second longitudinal section of the outer tube, which has at least one cross-sectional constriction element, is arranged between the combustion chamber and the first longitudinal section of the outer tube. In this way, the cross-sectional constriction element of the heat exchange tube influences the flow of the gas in its inlet region and increases the flow velocity and turbulence within the heat exchange tube.

It goes without saying that the features described above and those yet to be explained below can be used not only in the respectively stated combination but also in other combinations or individually without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the subject matter of the invention emerge from the following description in conjunction with the drawing, in which preferred exemplary embodiments of the invention are shown by way of example.

In the attached figure:

FIG1 shows a perspective view of a heating boiler according to the present invention;

FIG2 shows another perspective view of the heating boiler with a partially cutaway housing;

FIG3 is a perspective view showing a detail of a heat exchange tube according to the present invention of a heating boiler;

FIG4 is a cross-sectional view of a heat exchange tube according to the present invention;

FIG5 shows a perspective view of a heat exchange tube according to the present invention;

FIG6 shows a side cross-sectional view of a heat exchange tube according to the present invention along the tube plane;

FIG7 shows another side cross-sectional view of the heat exchange tube according to the present invention along another tube plane;

FIG8 shows an enlarged view of a longitudinal section of the heat exchange tube from FIG6 ;

FIG9 shows another enlarged view of the longitudinal section of the heat exchange tube from FIG7 ;

FIG10 shows a cross-sectional view of a heat exchange tube according to the present invention in an axial position;

FIG11 shows another cross-sectional view of the heat exchange tube according to the present invention for another axial position;

FIG. 12 shows a cross-sectional view of the heat exchange tube according to the position shown in FIG. 10 , wherein the through-flow cross section can be seen; and

FIG. 13 shows a perspective view of a heat exchange tube according to the invention, wherein the region of the cross-sectional constriction element is shown in section.

Specific implementation plan

FIG1 shows a perspective view of the housing 1 of a heating boiler 2. In FIG2 , the housing 1 is partially omitted to provide a better view of the interior of the housing 1. The heating boiler 2 is used to heat hot water in a heat circuit (not shown) and can be implemented as a condensing boiler. The housing 1 surrounds a hot water chamber 3 and includes a combustion chamber 4 of pot-shaped or conical design, which is arranged above the hot water chamber 3 and has an associated burner (not shown). A heat exchanger is arranged at the bottom of the combustion chamber 4. The heat exchanger has a plurality of heat exchange tubes 5 that extend through the hot water chamber 3 and into an exhaust gas collection chamber (not shown). Thus, the heat exchange tubes 5 extend from the bottom of the combustion chamber 4 and, in the illustrated embodiment, extend approximately vertically through the hot water chamber 3. Alternatively, any angle between a horizontal orientation of 0° and a vertical orientation of 90° within the hot water chamber is conceivable. The outer surfaces of the heat exchange tubes 5, through which hot water flows, dissipate their heat to the hot water in the hot water chamber 3. A temperature gradient exists within the heat exchange tubes 5, resulting in a significantly higher temperature in the upper region than in the lower region. Return connections 6 or 7 lead to the hot water chamber, through which cooled return water from different heat circuits is fed back to the hot water chamber 3. The heat circuit connected to return connection 6 is used, for example, for heating service water (i.e., has a relatively high return temperature), while lower return connection 7 is connected to a heat circuit used, for example, for floor heating (i.e., has a relatively low return temperature). Warmed hot water for the heat circuits is drawn through upper supply connection 8.

FIG2 shows a heat exchanger tube 5, which is designed according to the invention with an indentation or, respectively, a circumferential indentation 9 in its upper region. The perspective detail view of FIG3 shows each heat exchanger tube 5 according to the invention. As can be seen, the heat exchanger tube 5 comprises an outer tube 10 and a profile insert 11 that is inserted into the outer tube 10 in the assembled state. During operation of the heating boiler 2, exhaust gas from the boiler combustion flows through the outer tube and hot water surrounds the outer tube. In the illustrated embodiment, the outer tube 10 is formed from a metal alloy, preferably steel. The profile insert 11 has ribs 14 extending in its longitudinal direction 12 to increase the inner surface of the outer tube 10 and is in thermal contact with the outer tube 10. To improve heat transfer, the profile insert 11 is formed from aluminum.

In the illustrated embodiment, the profile insert 11 comprises a tubular body composed of two shell elements 15 and 16. Both shell elements 15 and 16 have a semicircular cross-section. A one-piece profile insert 11 is also conceivable, but this is not cost-effective to manufacture. Instead, the goal is to create at least a two-part profile insert 11, whose shell elements are designed in a sector-shaped manner, thereby creating a closed profile insert 11. According to this embodiment, the tubular body comprises two shell elements 15 and 16, which are provided with groove-shaped recesses 18 and rib-shaped projections 19 on their contacting longitudinal edges 17 and thus engage in a sealing manner, as shown in the enlarged detail in FIG. 4 . Both shell elements 15 and 16 are provided on their inner sides with ribs 14 extending into the inner cross-section of the tubular body and in the longitudinal direction 12 of the outer tube 10. Each shell element 15 and 16, with its rib 14, forms a profile that is open on one side. In particular, the two shell elements 15, 16 are each formed with a recess 18 serving as a sealing groove on one longitudinal edge 12, and a sealing rib adapted to the shape of the sealing groove, which is a projection 19, is formed on the other longitudinal edge 12. The profile insert 11 joined by the two shell elements 15, 16 directly bears against the outer tube 10 over its entire circumference and is manufactured with an outer diameter slightly smaller than the inner diameter of the outer tube 10, so that the profile insert 11 can be smoothly inserted into the outer tube 10.

As can be seen from FIG. 3 , the outer tube 10 and the profiled insert 11 have different axial lengths, which is illustrated in FIGS. 6 and 7 , which show different side views of a heat exchange tube 5 according to the invention, whereas FIG. 5 shows a single heat exchange tube 5 in which the profiled insert 11 has been pushed into the outer tube 10 and is not visible from the outside.

As shown in FIG6 , the axial length 20 of the outer tube 10 ideally corresponds to 1.5 times the axial length 21 of the profile insert 11. It is also conceivable that the axial length 20 of the outer tube 10 corresponds to 1.3 or 1.7 times the axial length 21 of the profile insert 11. The different axial lengths 20, 21 of the outer tube 10 and the profile insert 11 result in the outer tube 10 being divided into two longitudinal sections. The first longitudinal section 22 of the outer tube 10 is cylindrical and smooth-walled. The second longitudinal section 23 of the outer tube 10 has at least one cross-sectional constriction element 24 that narrows the flow cross section. The profile insert 11 extends only over the first longitudinal section 22 of the outer tube 10. This means that, in the illustrated embodiment, the axial length 25 of the first longitudinal section 22 corresponds to at least twice the axial length 26 of the second longitudinal section 23. As an alternative length ratio, in very specific applications, the axial length 26 of the second longitudinal section 23 can also be greater than the axial length 25 of the first longitudinal section 22.

FIG6 shows that the profile insert 11 does not end flush with the outer tube 10, but is pushed a short distance into the outer tube 10, so that the profile insert 11 is completely accommodated by the outer tube 10 and in particular by the first longitudinal section 22. Furthermore, FIG6 in conjunction with FIG2 shows that the second longitudinal section 23 of the respective outer tube 10, which has the cross-sectional constriction element 24, is arranged between the combustion chamber 4 and the respective first longitudinal section 22 of the corresponding outer tube 10. The respective cross-sectional constriction element 24 is therefore arranged directly downstream of the combustion chamber 4.

Here, the cross-sectional constriction element 24 can be formed as a nozzle-like tube insert that is pushed into the second longitudinal section 23 of the outer tube 10. As a result, the outer tube 10 can be designed continuously with smooth walls both in the first and second longitudinal sections 22, 23. In contrast, in the illustrated embodiment, the second longitudinal section 23 of the outer tube 10 has an indentation or indentation 9.

The shape of the cross-sectional constriction element 24 will now be described in greater detail in the overviews of Figures 6 to 13. For this purpose, the cross-sectional area of the tube 10 is divided according to Figures 6, 7, and 10 with reference to a first tube plane 27 and a second tube plane 28 extending perpendicularly thereto. Figure 6 shows a cross-sectional view along the first tube plane 27, while Figure 7 shows a cross-sectional view along the second tube plane 28. As shown in Figures 6 to 13, the cross-sectional constriction element 24 includes two first indentations or press-in sections 29, 30, which are formed in the wall of the second longitudinal section 23 of the outer tube 10. In particular, the first indentations 29, 30 are pressed into the wall of the second longitudinal section 23, resulting in the first indentations 29, 30 being concave or inwardly curved. The two first indentations 29, 30 are arranged diametrically opposed and are configured mirror-symmetrically with respect to the first tube plane 27. A first flow gap 31 (see FIG. 8 ) is formed between the two first indentations 29 , 30 and is between 2% and 3% of the diameter 32 of the outer tube 10 (see FIG. 6 ), as shown in FIG. 8 , which shows an enlarged view of section A in FIG. 6 . To form the first indentations 29 , 30 , the wall of the outer tube 10 is pressed inwards at points on both sides of the tube, thereby forming inwardly curved indentations that form the first flow gap 31 at their smallest distance. The walls of the indentations 29 , 30 are deformed over an axial length 33 (see FIG. 9 ) that corresponds to 0.4 times the axial length 26 of the second longitudinal section 23 , although an axial length 33 of 0.3 to 0.5 times the axial length 26 is also possible. Here, the wall is pressed onto the deformed axial length 33 for the first setbacks 29 , 30 as a whole in such a way that, over the axial length 33, the wall has a maximum diameter 34 for the first setbacks 29 , 30 which corresponds to 0.6 times the diameter 32 of the smooth-walled outer tube 10 , wherein a maximum diameter 34 of 0.5 to 0.7 times the diameter 32d of the smooth-walled outer tube 10 is also possible.

Figures 7 and 9 show further views of the second ridges 35 and 36, wherein the axial length 33 of the deformation is ideally the same for the first ridges 29 and 30 and the second ridges 35 and 36 and is therefore only shown in Figure 9. However, the axial length of the deformation can also be different for the first and second ridges. Furthermore, Figure 7 shows an axial section F-F in addition to the profile insert 11, which is shown in Figure 11 and depicts the outer tube 10 and the two shell elements 15 and 16 that form the profile insert 11. The two second ridges 35 and 36, together with the two first ridges 29 and 30, form the cross-sectional constriction element 24, wherein the first ridges 29 and 30 are designed differently from the second ridges 35 and 36. The two second setbacks 35 and 36 are also arranged diametrically opposite each other, and are designed to be mirror-symmetrical with respect to the second tube plane 28. Second indentations 35, 36 are also pressed into the wall of the second longitudinal section 23, resulting in a concave or inwardly curved indentation. A second flow gap 37 is formed between the two second indentations 35, 36. This second flow gap 37 is larger than the first flow gap 31 and is between 18% and 22% of the diameter 32 of the outer tube 10 (see FIG. 6 ), as shown in the enlarged view of section B in FIG. 7 in FIG. 9 . To form the second indentations 35, 36, the wall of the outer tube 10 is also pressed in at points on both sides of the tube, forming inwardly curved indentations that form the second flow gap 37 at their smallest distance. The walls of the indentations 35, 36 are deformed over an axial length 33 (see FIG. 9 ), which also corresponds to 0.4 times the axial length 26 of the second longitudinal section 23. Axial lengths 33 of 0.3 to 0.5 times the axial length 26 are also possible. To produce the second indentations or setbacks 35 , 36 , the wall is pressed onto the axial length 33 as a whole in such a way that, over the axial length 33 , the wall has a maximum diameter 38 of the second setbacks 35 , 36 that corresponds to 0.55 times the diameter 32 of the smooth-walled outer tube 10 , wherein a maximum diameter 38 that corresponds to 0.45 to 0.65 times the diameter 32 of the smooth-walled outer tube 10 is also possible.

The previously described configuration of first setbacks 29, 30 and second setbacks 35, 36 results in a flow cross section 39, which is shown in FIG10 by the shaded area representing the profile insert 11 formed by shell elements 15, 16, and in FIG12 by the black area. Because the first and second bulges 29, 30, 35, 36 are formed at the same axial position in the second longitudinal section 23 of the outer tube 10, i.e., both first and second bulges 29, 30, 35, 36 extend over the same axial length 33, the flow cross section 39 formed by the first and second bulges 29, 30, 35, 36 in the second longitudinal section 23 of the outer tube 10 has an H-shaped cross section. FIG13 shows the outer tube 10, with the tube section beginning at the H-shaped cross section omitted so that the H-shaped flow cross section 39 can be more clearly seen.

In the heat exchanger tube 5 according to the present invention, the cross-sectional constriction element 24 of the outer tube 10 is a doubly symmetrical constriction, which avoids the disadvantages known from the prior art. A problem with the heat exchanger tubes known from the prior art is that the hot gas flows through the heat exchanger tube from its inlet to its outlet and cools there. The associated significant reduction in gas volume leads to a significant reduction in the flow velocity and turbulence up to the heat exchanger tube outlet, which negatively affects the heat exchange performance. The present invention improves heat exchange because the flow velocity and turbulence in the heat exchanger tube 5 according to the present invention are increased by the cross-sectional constriction element 24. The indentations or setbacks 29, 30, 35, 36 reduce the pressure loss in the upstream region upstream of the indentations or setbacks 29, 30, 35, 36. This allows significantly more energy to be exchanged in the combustion chamber 4 and in the tube section of the heat exchanger tube 5 upstream of the setbacks 29, 30, 35, 36. In the region of the indentations 29, 30, 35, 36, the constriction elements significantly increase the flow velocity, thereby also increasing the heat exchange and, therefore, the energy utilization. In the region behind the indentations 29, 30, 35, 36 (i.e., downstream of the constriction elements), the exhaust gas expands again and is guided through the section with the profile insert 11. Here, the very large surface area of the ribs 14 of the profile insert 11 is utilized to cool the exhaust gas down to below the dew point, thus contributing to the advantages of condensing boiler technology.

The main advantages of the present invention can be summarized as follows:

The reduction in pressure loss leads to a better heat exchange in the combustion chamber 4 and at the inlet of the heat exchange tubes 5 .

The increase in the flow velocity in the region of the constriction 24 or the indentations 29 , 30 , 35 , 26 leads to a better heat exchange (laminar flow versus turbulent flow).

For low flow velocities in the first longitudinal section 22 of the heat exchange tube 5 downstream of or following the constriction 24 and for low exhaust gas temperatures, the increase in the heat exchange surface achieved by means of the ribs 14 of the profile insert 11 leads to better heat exchange.

By using the heat exchange tubes 5 according to the present invention in the heating boiler 2 , 85% to 90% more energy can be exchanged compared to the previously known technology.

The invention described above is obviously not limited to the embodiments described and illustrated. It is obvious that numerous modifications readily achievable by a person skilled in the art, depending on the intended application, can be implemented with respect to the embodiment illustrated in the drawings without departing from the scope of the invention. For example, the cross-sectional constriction element 24 (instead of four indentations) could be configured as a single indentation 9 in the wall of the second longitudinal section 23 of the outer tube 10, or multiple cross-sectional constrictions could be arranged one behind the other in the axial direction 12 or at different axial tube positions, with the respective indentations 9 being arranged one behind the other. All matters contained in the description and/or illustrated in the drawings, including deviations from the specific embodiments, readily achievable by a person skilled in the art, are included in the present invention.

Claims (15)

1. A heat exchange tube (5) of a heating boiler (2), the heat exchange tube having an outer tube (10) and a shaped insert (11) pushed into the outer tube, wherein exhaust gas from boiler combustion can flow through the outer tube and hot water can surround the outer tube on the outside, the shaped insert having ribs (14) extending in its longitudinal direction (12) to increase the inner surface of the outer tube (10) and being in thermally conductive contact with the outer tube (10), The first longitudinal section (22) of the outer tube (10) is constructed in the form of a smooth cylindrical wall, while the second longitudinal section (23) of the outer tube (10) has at least one cross-sectional narrowing element (24) that narrows the flow cross-section. The irregularly shaped insert (11) extends only on the first longitudinal section (22) of the outer tube (10). The at least one cross-sectional narrowing element (24) includes at least two first recesses (29, 30) constructed in the wall of the second longitudinal section (23) of the outer tube (10), wherein the two first recesses (29, 30) are arranged diametrically opposite each other and are constructed in a mirror-symmetrical manner about the first tube plane (27), and The cross-sectional narrowing element (24) attached to the at least two first indentations (29, 30) also includes at least two second indentations (35, 36), which are formed by the walls of the second longitudinal section (23) of the outer tube (10). The two second indentations (35, 36) are arranged opposite each other in diameter and are mirror-symmetrical about a second tube plane (28) extending perpendicular to the first tube plane (27). 2. The heat exchange tube (5) according to claim 1, characterized in that at least one first flow gap (31) is constructed between the at least two first recesses (29, 30), the first flow gap being between 2% and 3% of the diameter (32) of the outer tube (10). 3. The heat exchange tube (5) according to claim 1, characterized in that at least one second flow gap (37) is constructed between the at least two second recesses (35, 36), the second flow gap being between 18% and 22% of the diameter (32) of the outer tube (10). 4. The heat exchange tube (5) according to claim 3, characterized in that the first and second raised portions (29, 30, 35, 36) formed by the first and second recessed portions and raised into the interior of the outer tube (10) are constructed at the same axial position in the second longitudinal section (23) of the outer tube (10), wherein the flow-through cross section (39) of the second longitudinal section (23) of the outer tube (10) constructed by the first and second raised portions (29, 30, 35, 36) has an H-shaped cross section. 5. The heat exchange tube (5) according to any one of claims 1 to 4, characterized in that the axial length (25) of the first longitudinal section (22) is at least twice the axial length (26) of the second longitudinal section (23). 6. The heat exchange tube (5) according to any one of claims 1 to 4, characterized in that the irregular insert (11) includes a tube body, the tube body being composed of at least two shell elements (15, 16) each having a fan-shaped cross-section. 7. The heat exchange tube (5) according to claim 6, characterized in that the tube body comprises two shell elements (15, 16), the two shell elements having groove-shaped recesses (18) and rib-shaped protrusions (19) on their longitudinal edges (17) where they contact each other, and thus being sealed to each other, wherein the two shell elements (15, 16) have ribs (14) extending into the inner cross-section of the tube body and along the longitudinal direction (12) of the outer tube (10) on their inner sides, such that each shell element (15, 16) forms a one-sided open shape with its ribs (14). 8. The heat exchange tube (5) according to claim 7, characterized in that the two shell elements (15, 16) are respectively provided with a sealing groove (18) on one longitudinal edge (17) and a sealing rib (19) matching the shape of the sealing groove (18) on the other longitudinal edge (17). 9. The heat exchange tube (5) according to claim 1, characterized in that the at least one cross-sectional narrowing element (24) is formed as a tube insert constructed in the form of a nozzle, the tube insert being pushed into the outer tube (10) and into its second longitudinal section (23). 10. The heat exchange tube (5) according to any one of claims 1 to 4 and 9, characterized in that the outer tube (10) is formed of a metal alloy, and the irregular insert (11) is formed of aluminum. 11. The heat exchange tube (5) according to any one of claims 1 to 4 and 9, characterized in that the heat exchange tube (5) of the heating boiler (2) is a heat exchange tube of a condensing boiler. 12. The heat exchange tube (5) according to any one of claims 1 to 4 and 9, characterized in that the outer tube (10) is formed of steel and the shaped insert (11) is formed of aluminum. 13. A heating boiler (2) for heating hot water in a heat circulation loop, the heating boiler having a shell (1) defining a hot water chamber (3) and having a combustion chamber (4) preceding the hot water chamber (3). Its features are, At least one heat exchange tube (5) according to any one of claims 1 to 12 is arranged inside the housing (1), the heat exchange tube leaving the combustion chamber (4) and extending through the hot water chamber (3). 14. The heating boiler (2) according to claim 13, wherein a second longitudinal section (23) of the outer tube (10) having at least one cross-sectional narrowing element (24) is arranged between the combustion chamber (4) and the first longitudinal section (22) of the outer tube (10). 15. The heating boiler (2) according to claim 13 or 14, wherein the heating boiler (2) is a condensing boiler.